It is known that vehicles having an electric drive which has an electric motor can be braked in a generator mode (which may include a so-called regenerative mode). During the generator-type braking, the electric motor is operated as a brake by operating the electric motor as a generator.
It is also known that this generator-type braking is switched off as soon as the vehicle movement dynamics control system becomes active. For example, the vehicle movement dynamics control system can be an anti-lock brake system (ABS), an electronic stability program (ESP), an ESC (electronic stability control) or a traction control system (TCS). The vehicle movement dynamics control then takes place only via one or more friction brakes. Although a friction brake can generate a high friction braking torque, the friction brake does not have sufficient dynamics to be able to set itself to a coefficient of friction of the road which changes quickly or suddenly. However, this means that the vehicle movement dynamics control system can no longer keep a wheel slip in a stable range. This has an adverse effect on the stability of the vehicle with respect to the driving behavior. That is to say the vehicle can, under certain circumstances, become dynamically unstable.
A further disadvantage is that the activation of the friction brake results in increased abrasion, which results in an increased quantity of fine friction material dust.
The object of the invention is therefore to provide a method for performing closed-loop control of a wheel brake slip and a wheel brake slip control system for a vehicle having an electric drive operated under control of a computer program, which overcome the above disadvantages.
The object is achieved by means of a method for performing closed-loop control of a wheel brake slip in accordance with this invention, by means of a wheel brake slip control system and by means of a computer program.
Preferred embodiments of the invention are the subject matter of this specification and claims.
The method according to the invention for performing closed-loop control of a wheel brake slip comprises the idea that when a vehicle movement dynamics control system becomes active, it is carried out not only by means of a friction brake but also with the aid of torque generated by the electric drive. The electric drive signals and friction brake signals which are generated by means of the slip control device correspond here, in particular, to setpoint values or target values. The electric drive generates an electric drive torque which can be positive or negative, wherein the electric drive comprises an electric motor. A positive electric drive torque causes the wheel to be driven. A negative electric drive torque causes the wheel to be braked. In the latter case, the electric motor is operated as a generator. A negative electric drive torque is also referred to below as a generator-type (regenerative) braking torque. The overall braking torque which is required for slip control is therefore obtained as the sum of the electric drive torque and of the friction braking torque, wherein the changes in torque which are fast but relatively small in amplitude are generated by the electric drive, and the remaining braking torques are generated by the friction brake. In particular, the friction brake generates a basic braking torque onto which the electric drive torque is applied and modulated. In the braking method according to the invention, a slip control is therefore carried out by means of the friction brake and the electric drive, for which reason the slip control can also be referred to as a cooperative slip control.
The advantages of this cooperative slip control arise, in particular, by virtue of the fact that the electric drive can quickly change its generated electric drive torque, and this torque which can be controlled quickly but is limited in magnitude ideally supplements the braking torque of the friction brake which can be controlled more slowly but is larger in absolute value. As a result, the vehicle movement dynamics control can be advantageously set to quickly changing vehicle situations, in particular if a coefficient of friction of the road changes.
If a vehicle with an electric drive is mentioned below, this is also intended to include electric drives which have more than one electric motor. In particular, one electric motor may be provided per wheel of a vehicle. That is to say each wheel can be driven or braked individually by means of an electric motor. However, it is also possible to provide that the vehicle has just one electric motor per axle, wherein driving or braking of the wheels takes place here in particular by means of a differential gear mechanism. In particular, in the latter case the torques which are required to perform slip control of the two wheels of the axle is divided into a common highly dynamic component, which is generated by means of the electric motor, and into two further wheel-specific components which are generated by means of the respective friction brakes of the wheels. The vehicle axle can also have, for example, double wheels. Furthermore, the vehicle can comprise not only an electric drive but also, for example, additionally comprise a gasoline engine or a diesel engine. Such vehicles are usually also referred to as hybrid vehicles.
According to one advantageous refinement of the invention it is possible to provide that an electric drive speed signal is measured and evaluated by means of an evaluation device which is connected to the slip control device. For example, the evaluation device can also be integrated into the slip control device. An electric drive speed signal can be generated, in particular, by means of an electric motor angle sensor (or shaft encoder), for example an electric motor position sensor, which is arranged, for example, in the electric motor for an internal magnetic field control. An electric motor speed which corresponds to a wheel speed of the wheel which is connected to the electric motor can therefore be advantageously measured more quickly and more precisely than in conventional slip control systems.
It can preferably be provided that a highly dynamic control process of the electric drive torque is carried out at least partially in the electric drive control device. The electric drive control device detects and evaluates, in particular, an electric drive speed. In particular, the electric drive control device comprises a magnetic field control for the electric motor. It is therefore advantageously made possible for the highly dynamic control of the braking effect of the electric motor to take place entirely or partially in the same electronic control device as the motor speed evaluation for the magnetic field control of the electric motor.
In another preferred embodiment of the invention, a friction brake torque is set to a smaller value than an electric drive torque if a generator-type braking torque which can be generated by means of the electric drive torque is greater than a braking torque which can be applied to a roadway. In such a driving situation, the generator-type braking effect alone is sufficient to perform closed-loop adjustment of the wheel to an optimum brake slip. In particular, under certain circumstances it is even possible here to dispense with activation of the friction brake, i.e. for the friction braking torque to be equal to 0. The friction brake therefore does not have to be activated. As a result, brake lining wear can be advantageously considerably reduced. Fine dust emissions and CO2 emissions are therefore also clearly reduced. The distance or an air gap between a brake caliper and a brake disc of the friction brake is preferably set to zero. This means that the brake caliper on the brake disc slips to just such an extent that no friction braking effect is achieved. The friction brake is to this extent not in its position of rest. This has, in particular, the advantage that the friction brake can be activated directly and without a delay, for example if the coefficient of friction of the road changes.
According to the invention it is also possible to provide that electrical energy which is formed by means of the electric drive is at least partially converted into thermal energy if the electric drive torque is formed as a generator-type braking torque. In this context, in particular the kinetic energy which is generated during the braking process is at least partially fed as electrical energy to an electric resistor, in particular a braking resistor, for example a controlled braking resistor which then converts said electrical energy into thermal energy in the form of heat. This thermal energy can be conducted away, for example, by cooling or can be used as useful heat for further functions, for example heating a passenger compartment. The proportion of the electrical energy which is generated during the braking process and which is not converted into thermal energy is fed back, in particular, into an electrical energy store, with the result that this electrical energy can be made available to electrical consumers at later times. However, there may, in particular, also be provision that at least part of this proportion is fed directly to one or more electrical consumers. Reactive power can also preferably be generated in the electric motor by means of a control means of the electric drive without the electric drive torque changing. The additional electrical power loss which is generated as a result of this is carried away with the cooling of the motor and the windings in the electric motor therefore act as an integrated braking resistor.
The computer program according to the invention can be stored, for example, in a piece of firmware, wherein the firmware can be integrated, in particular, into the slip control device. The firmware is preferably implemented partially in the slip control device and partially in the electric drive control device.